Showing Revision 6 created 05/25/2016 by Udacity Robot.

1. Title:
   Add to Index Solution - Intro to Computer Science
2. Description:

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   There are different ways that we can define add to index,
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   here's one way that works.
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   We're going to create a procedure, we'll call it add to index--
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   and it takes our 3 inputs--the index, the keyword, and the URL--
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   What we need to do in add to index is first find if the keyword already appears.
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   To do that, we need to look through all of the entries in index.
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   The natural way to do that is to use a 4-loop.
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    We're going to define the procedure add to index--
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    and it takes our 3 inputs, index, keyword and URL--
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    to help keep track of what we are doing,
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    I am going to draw a reminder of what the data structure of index is.
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    Remember that it is a list of entries,
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    and each entry is itself a list,
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    where the 1st part is a keyword, and the 2nd part is a list of URLs.
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    That's our data structure,
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    that's going to help us figure out what to do to define add to index.
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    The first thing we need to do
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    is to check whether the keyword already exists in the index.
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    If we can find it, well, then we want to modify that entry, rather than creating a new one.
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    The natural way to do that is to use a 4-loop.
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    We are going to loop through the elements of index.
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    We will give each one the name entry to use in the block.
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    This is what entry will be.
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    The first time through the loop, the value of entry will be a reference to the list here--
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    which is the first element of the index list.
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    Now we need to find the keyword.
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    The keyword is right here--that's the element at position zero of entry.
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    We're going to test the value at position zero on entry
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    identical to the keyword that's passed in
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    we'll use the double-equal comparison to test that--
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    If it is equal, then we found a match--
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    this means we want to append the URL to the list of URLs associated with that entry.
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    To get that list of URLs
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    we want to find entry 1--that's the value at position 1 of entry--
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    and we want to append that the new URL.
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    Here we've found an entry that matches the keyword we were looking for--
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    this means the keyword is already in the index--
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    we've added the new URL to the URLs associated with that keyword--
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    so we're done, we have nothing else to do.
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    What we want to make sure, is that we don't continue and and do anything else.
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    One approach would be to use break---that would end the loop--
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    what we want to do instead is really end the whole procedure.
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    If we did break, well, then we'd still have the problem of how do we deal with the case
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    where the keyword wasn't found?
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    Here we're just going to return--
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    we're all done with add to index, we've added the URL it belongs.
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    Now we need to think about what to do in the case
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    where the keyword does not already exist in the index.
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    If that's the case, then we get to the end of the loop without ever finding that entry.
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    If we've got to the end of the loop,
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    that means we did not find any entry in the index that matches the keyword,
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    then what we want to do is add a new entry---
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    and that new entry is going to have, as its value,
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    a list containing 2 elements, it will be the keyword--
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    and as the 2nd element
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    we'll have a list containing the URLs that we found that have that keyword.
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    So far we only have 1--the URL that was passed in to add to index.
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    How do we do that?
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    To add a new element to add to index we use append.
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    We need something to pass in to append--that is the structure we want to add.
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    This whole thing is what we want to add--
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    so, that's a list containing the value keyword as its 1st element.
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    As the 2nd element, its a list containing just the single URL.
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    That's what we want to append.
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    in the case where we didn't already find the keyword in the index.